Adjustable speed drives are known for use in the control of multi-phase electric motors. In some cases, the state of an electric motor being driven by an adjustable speed drive may be unknown. Specifically, the angular position and/or the speed of rotation of a rotor of the motor may be unknown. This can happen for many different reasons. Such a situation can occur, for example, following a short power outage. Alternatively, this situation can arrive when a rotor is already spinning before motor drive control has commenced (i.e. on startup of the motor drive).
Thus, a motor may have both mechanical and electrical energy stored therein before being under the control of a motor drive. Algorithms are known that seek to enable a motor drive to take control of a motor without creating a large disturbance in the energy stored in the machine and to do so without taking all the energy out of the motor. However, there remains a need for improved and alternative motor control systems and methods.
The present invention seeks to provide an alternative method to those already available.